Many types of antennas are in wide use today throughout the communications industry. The antenna has become an especially critical component for an effective wireless communication system due to recent technology advancements in areas such as Personal Communications Services (PCS), cellular mobile radiotelephone (CMR) service, and Advanced Mobile Phone System (AMPS) service.
Some conventional PCS, CMR, and AMPS systems can use vertically or horizontally, singularly polarized antennas to transmit and receive RF communications. An example of such a conventional system is illustrated in FIG. 1A. In this Figure, spatial separation is used between the three antenna arrays 100A, 100B, and 100C in order to avoid electrical interference and thus increase electrical isolation between each antenna array 100.
In the exemplary conventional system illustrated in FIG. 1A, single polarization transmitting or receiving antenna arrays 100 can be separated by distances 105 having a magnitude such as on the order of approximately ten wavelengths. This means that individual receiving or transmitting antenna elements 102 of one antenna array 100 would be separated from another like antenna array 100 by a distance of approximately ten wavelengths.
While this physical separation between like antenna arrays 100 can reduce electrical interference and increase electrical isolation, this arrangement is often not practical given the tight spacing and electronic packaging requirements imposed on most antennas. That is, physical separation between antenna arrays and/or antenna elements is often not possible when antennas are required to occupy a space or volume that may be smaller than the optimal antenna wavelength separation.
To address small space or volume requirements, dual polarized antennas can be used. Specifically, a crossed dipole pair radiator having two radiating sub-elements that are polarity specific to transmit and receive RF signals at two different polarizations can be employed. In a conventional crossed dipole pair antenna, such as illustrated in FIG. 1B, the dipoles for each polarization of a respective crossed dipole pair dual polarized antenna array 115 are usually collocated or very close to each other so that there is essentially no physical separation at all between transmitting and receiving antenna elements. In the antenna system illustrated in FIG. 1B, a duplexer 120 can be used to switch between transmitted and received RF signals.
The dual polarization antenna illustrated in FIG. 1B is prevalent in the wireless communications industry due to the polarization diversity properties that are inherent in this type of antenna. This type of crossed dipole pair dual polarized antenna can increase the antenna's signal handling capacity and can mitigate the deleterious effects of fading and cancellation that often result from today's complex propagation environments.
Dual polarized antennas in general are usually designed in the form of an array antenna and have a feed network associated with each of the two dipoles of the crossed dipole pair. A dual polarized antenna is usually characterized by having two antenna connection terminals or ports for communicating signals to the antenna that are to be transmitted, and for outputting signals from the antenna that have been received. Thus the connection ports serve as both input ports and as output ports at any time, or concurrently, depending on the antenna's transmit or receive mode of operation.
An undesirable leakage signal can appear at one of these ports as a result of a signal present at the opposite port and part of that signal being electrically coupled, undesirably so, to the opposing port. This coupling can occur when stray radiation from one antenna element is detected by the opposing antenna element. A leakage signal can also be produced by self-induced coupling when a signal propagates through a feed network.
The measuring of leakage signals in a dual polarized crossed dipole antenna is illustrated in the conventional art of FIG. 1C. A main transmission signal al can be supplied at port 35. This transmission signal al is propagated by the antenna elements 11 coupled to port 35 when these antenna elements 11 are operating in a transmit mode. An undesirable leakage signal b1 can be measured at port 35 as a result of the transmission signal a1 exciting portions of the feed network such as distribution network 15.
In another example, the undesirable leakage signal b1 can be measured at port 35 when a transmission signal a2 is supplied at port 40. The transmission signal a2 can excite portions of the feed network such as distribution network 17 which in turn, can excite antenna elements 11, 12 or distribution network 15 or both. It is noted that other leakage signals (not shown) may be measured at port 40 which are caused by transmission signal a2 itself or RF signals supplied at port 35.
A dual polarized antenna's performance in terms of it transmitting an RF signal with low antenna loss of the signal, or of it receiving an RF signal and having low antenna loss at the antenna's output received signal, can be measured in large part by the signals' electrical isolation between the antenna's two connection ports, i.e., the port-to-port isolation at the connectors or the minimizing of the leakage signal b1. Dual polarized antennas can also have radiation isolations defined in the far-field of the antenna which differ from port-to-port isolations defined at the antenna connectors. The focus of the invention described in detail later in this document is not on far-field isolation, but rather with port-to-port isolations at connector terminals of a dual polarized antenna.
While a dual polarized antenna can be formed using a single radiating element, the more common structure is an antenna having an array of dual polarized radiating elements 10. In practice, both the transmit and receive functions often occur simultaneously and the transmit and received signals may also be at the same frequency. So there can be a significant amount of electrical wave activity taking place at the antenna connectors, or ports, sometimes also referred to as signal summing points.
The effect of the significant amount of electrical wave activity during simultaneous transmission and reception of RF signals can be explained as follows. Poor receive sensitivity, and poor radiated output, often results due to internal antenna loss when part of one of the signals at one input port (port one) leaks or is otherwise coupled as a leakage signal to the other port (port two). Such leakage or undesired coupling of a signal from one port to the other may adversely combine with the signal at the other port to diminish the strength of both signals and hence reduce the effectiveness of the antenna.
When port-to-port isolation is minimal, i.e., leakage is maximum, the antenna system will perform poorly in the receive mode in that the reception of incoming signals will be limited only to the strongest incoming signals and lack the sensitivity to pick up faint signals due to the presence of leakage signals interfering with the weaker desired signals. In the transmit mode, the antenna performs poorly due to leakage signals detracting from the strength of the radiated signals.
Adding to the complexity of electrical wave activity during simultaneous transmission and reception of RF signals with dual polarized antennas is the positioning of dual polarized antennas operating in different frequency bands. Currently, there is a trend in the conventional art towards using dual band antennas in close proximity with one another which cover two frequency bands (a high frequency band and a low frequency band) within one mechanical package.
For example, as illustrated in FIG. 1D, an antenna array 117 can comprise high frequency band antenna elements 115B and low frequency band antenna elements 115A. As understood by one of ordinary skill in the art, the high frequency band antenna elements 115B have resonant dimensions that are smaller when compared to the low frequency band antenna elements 115A. A dual band, crossed dipole dual polarized antenna array 117 can further complicate the isolation problem because there can be interference between the two orthogonal radiated fields in a single frequency band, as well as interference between the high frequency and low frequency band antenna elements 115A, 115B.
Conventional Isolation Techniques
One known technique for minimizing this leakage signal problem is by incorporating proper impedance matching within the distribution networks that generate the two sets of RF signals. Impedance mismatching can cause leakage signals to occur and degrade the port-to-port isolation if (1) a cross-coupling mechanism is present within the distribution network or in the radiating elements, or if (2) reflecting features are present beyond the radiating elements. Proper impedance matching can minimize the amount of impedance mismatch that a signal experiences when passing through a distribution network, thereby increasing the port-to-port isolation.
In general, when impedance mismatches are present, part of a signal is reflected back and not passed through the area of impedance mismatch. In a dual polarized antenna system, the reflected signal can result in a leakage signal at the opposite port or the same port and it can cause a significant degradation in the overall isolation characteristic and performance of the antenna system. While impedance matching helps to increase port-to-port isolation, it falls short of achieving the high degree of isolation that is now required in the wireless communications industry.
Another technique for increasing the isolation characteristic is the physical separation of transmitting and receiving antenna elements as noted above and as illustrated in FIG. 1A. Individual radiating elements of an antenna array can be positioned sufficiently apart on the order of wavelengths in order to increase antenna isolation. However, as noted above, the physical area and dimensional constraints placed on the antenna designs of today for use in cellular base station towers generally render the physical separation technique impractical in all but a few instances.
Another technique for improving an antenna's isolation characteristic is to place a physical wall between each of the radiating elements. Still another is to modify the ground plane of the antenna system so that the ground plane associated with each port is separated by either a physical space or a non-conductive obstruction that serves to alleviate possible leakage between the two signals otherwise caused by coupling due to the two ports sharing a common ground plane. These techniques can help in increments, but usually do not solve the magnitude of the signal leakage problem.
Still another conventional technique for improving the isolation characteristic of an antenna is to use a feedback element to provide a feedback signal to pairs of radiators in the antenna array. The feedback element can be in the form of a conductive strip placed on top of a foam bar that can be positioned between crossed dipole radiators.
While the conductors, according to this technique, can increase the isolation characteristic, the foam bars that support the conductive strips positioned between crossed dipole pair antennas can have mechanical properties that are not conducive to the operating environment of the antenna. For example, the foam bars are typically made of non-conducting, polyethylene foam or plastic. Such materials are usually bulky and are difficult to accurately and precisely position between antenna elements.
Additionally, these support blocks have coefficients of thermal expansion that are typically not conducive to extreme temperature fluctuations in the outside environment in which the antenna functions, and they readily expand and contract depending on temperature and humidity. In addition to the problems with thermal expansion, the support blocks are also not conducive for rapid and precise manufacturing. Furthermore, these types of support blocks do not provide for accurate placement of the conductive strips or feedback elements on the distribution network board.
Consequently, there is a need in the art for a method and system that facilitates the design of a dual polarized antenna system, and specifically, a crossed dipole antenna pair, with a high degree of isolation between two respective antenna connection ports that more thoroughly cancels out any port-to-port leakage signals and at the same time, is conducive to high speed manufacturing and a high degree of accurate repeatability. There is also a need in the art for an antenna isolation method and system that can withstand extreme operating environments in which a cellular base station antenna is exposed.